Neuroinflammation is now regarded to be a contributing factor in the etiology of a wide range of psychiatric disorders and a potential mediator of the extensive co-morbidities that are present between these disorders. It has been suggested that psychiatric disorders may result when normal neuroinflammatory responses to stimuli that induce these responses become exaggerated. The experience of both acute and more chronic stressors also are associated with the development of a number of disorders. It has been tempting to suppose that these two processes are related, but neither acute nor chronic stress have proved to produce persistent neuroinflammation beyond the stressor exposure. However, we have recently found that both acute and chronic stressors, even though they do not produce either large or long-lasting neuroinflammation, potently exaggerate neuroinflammatory responses to both peripheral and central inflammatory stimuli that are administered later. Importantly, this sensitized neuroinflammatory reaction persists for many days after stressor exposure. However, the mechanisms that lead stressors to sensitize subsequent neuroinflammation remain largely unknown. Within the past decade there has been a revolution in understanding the mechanisms involved in mediating peripheral innate immunity/inflammation. These new mechanisms and processes have been studied almost exclusively in the periphery, and whether or not they occur in the CNS is unknown. Our Preliminary Studies strongly encourage the possibility that these are present in CNS innate immune cells (microglia) and that they are involved in mediating stress-induced sensitization of neuroinflammatory responses to subsequent inflammatory challenges. The global goals of the proposed research are to a) firmly establish the presence of these processes, heretofore unstudied in brain, in brain, and b) explore the role of these processes in stress- induced sensitization of neuroinflammation, as well as the behavioral changes that typically induced by the activation of innate immune cells in the brain.